This invention relates to a system for feeding web media from a supply roll at a controlled velocity.
There are numerous applications wherein a web media must be unwound from a supply roll and fed into a system which operates on the media in some way. Printers and plotters are known in the art which are capable of producing copies of large documents such as engineering drawings, blueprints and the like. An example of such a machine is the Xerox 2510. The image-recording media, typically paper, is supplied in roll form by winding the paper about an inner core thereby forming a roll assembly. The roll assembly is supported in an axial position so that the paper can be unrolled in a generally flat condition and is then fed into the paper path of the particular document reproduction system. A requirement for any media supply system, emphasized for a system reproducing large documents, such as 36 inch engineering drawings, is to move the paper at a constant, controlled velocity. Deviations from the optimum process velocity alter the system timing relationships resulting in undesirable deviations in the output.
Various prior art techniques are known to unwind web media from a supply roll at a constant velocity. One approach is to feed the paper into two pairs of drive rollers. Each roll pair consists of an elastomer-covered, high-friction drive roll and a hard idler roll. This arrangement is shown in FIG. 1. A supply system 2, shown in side view, allows web media 3 to be fed from a media supply roll 4. The leading edge of the web is fed between two pairs of rolls 5. Each roll pair comprises an elastomer, high friction drive roll 6 and a hard idler roll 7. Each roller pair defines a nip area at its interface. FIG. 2 is a graph which plots nip pressure distribution and surface velocity over nip area. For the case where there is zero drag on the media and idler roll 7, and if the coefficient of friction between media and drive roll is uniform throughout the nip, the following relationship exists: